THE OPEN-HEARTH FURNACE. 129 changed when the passages are clogged with dust. On the other hand, the loss of heat will also depend on these areas, for with larger orifices the gases will go through the checkers and to the stack without giving up their heat to the bricks, so that furnacemen must arrive at a compromise between large openings to allow long life to the checkers, and small openings to allow proper absorption of heat. There is also a third consideration, which is. to arrange the bricks in such a way that they present the maximum area of heat absorption with the least interference with the passage of the gases, and with the least opportunity for the deposition of dust on horizontal surfaces. The air chamber should be larger than the gas chamber, because a cubic foot of gas requires more than a cubic foot of air to attain FIG. VIII-C.—50-ToN CAMPBELL BASIC -FURNACE, STEELTON, PA.